Magnetic resonance imaging

ABSTRACT

Conventional gradient echo sequences with relatively long echo-times visualise complex flow as signal loss, the degree and extent of which is a qualitative indicator of valvular heart disease. Since the long echo-time precludes segmentation, breath-hold imaging is not possible and acquisitions typically take 2-minutes with respiratory motion artefact being a frequent problem. In the present invention, a segmented sequence is used which generates similar degrees of signal loss to a conventional gradient echo sequence. However, the acquisition duration is reduced and breath-hold imaging is feasible, removing respiratory motion artefact.

The present invention relates to magnetic resonance (MR) imaging, and inparticular to an improved imaging technique which enables more usefulresults to be obtained from imaging a patient.

As is well known, MR imaging (MRI) is a useful diagnostic tool whichenables analysis of internal structures and flows within a patient'sbody in non-invasive manner. In general terms, radio frequency waves areapplied to a patient's body in a region of changing magnetic field andthe molecules within the patient's body are caused to resonate atvarious frequencies which can be detected. The detected resonances areprocessed to enable imaging of internal structures, etc.

In general terms, the magnetic field is caused to alter in a number ofdifferent ways for different purposes in the imaging process. Inparticular a number of variations are superimposed onto a largeunderlying magnetic field which is caused to exist in the vicinity ofthe test volume.

Firstly, a ‘slice select’ magnetic field is applied such that themagnitude of the magnetic field changes along a first direction in aknown manner. Generally this direction is considered to be along thelength of the patient's body when present in the test volume. However,this is not necessarily the case. The purpose of this field is apparentwhen it is considered what happens when a radio frequency signal isapplied in the test volume. The resonant frequency of nuclei in thepatient's body depends upon the magnitude of the magnetic field aroundthem. Accordingly, for a given frequency of applied radio frequencysignal, the slice select field determines in which plane orthogonal tothe first direction nuclei will be excited. Accordingly, this field canbe said to select which ‘slice’ through a patient is excited by theapplication of the radio frequency energy pulse.

The excitation of the nuclei persists for a time after the applicationof the energy pulse, and during that time, measurements of theexcitations in the excited plane can be made. The time between theapplication of the energy pulse and the measurement is usuallydesignated TE. These measurements are made by a further coil placed nearto the patient which picks up electromagnetic signals from theoscillations of the nuclei.

As part of that reading process, a second magnetic field gradient isapplied in a second direction orthogonal to the first direction. Thesecond direction therefore lies in the plane of the selected slice. Thechanging magnetic field along one direction in the plane of the slicechanges the frequency response of the nuclei. The picked up signal inthe coil therefore includes a mixture of frequencies, which can beanalysed by Fourier analysis or the like to determine the magnitude ofthe responses at different locations in the second direction within theselected slice. This direction is known as the frequency encodedirection.

A third magnetic field gradient is applied in a third directionorthogonal to both of said first and second directions and is known asthe phase encoding gradient. This enables the locations of the resonantresponses to be determined in the third direction within the selectedslice, and so a full three dimensional picture of the resonant responsescan be established.

One common use of MRI is in imaging of hearts and blood flows throughthe heart which can be useful in the diagnosis and assessment ofpatients with different kinds of conditions of the heart. In the contextof imaging blood flows, and organs such as the heart which have a bloodthrough them, various techniques are well known to try to compensate forthe additional effects which the blood flow has on the imaging by virtueof its movement. It is for instance well known to have velocity oracceleration compensated imaging schemes.

Also, when imaging hearts, account has to be taken of the fact that theheart is constantly moving, and so it is well known to build up thefinal image from readings taken during a number of sequential cardiaccycles. It is possible to build up an image by taking a single phaseencoding in each cardiac cycle. This however takes a significant ofcardiac cycles (in excess of 100) to achieve a complete image. Thisleads to the further complication that additional movement of the heartis introduced by the breathing of the patient as the lungs are adjacentto the heart. This has in the past been addressed by the use oftechniques which enable multiple phase encoding steps to be done in eachcardiac cycle, thus reducing the number of cardiac cycles (to less than20) required to build up an image of a particular resolution. Inparticular it is possible to reduce the number of cardiac cyclesnecessary to build up an image such that it becomes feasible to ask thepatient to hold their breath for the duration of the imaging process,thereby minimising the respiratory artefacts in the image. Thistechnique is known as segmented imaging, interleaved imaging, orbreath-hold imaging.

The above outlined aspects of MRI are well known and the above is notintended to be exhaustive. For further background understanding,reference may be made to any standard paper on the subject or to earlierpatent applications in this field, for instance those of the presentinventors.

In cardiovascular MR, velocity compensated gradient echo sequences withrelatively long echo-times of around 15 ms have been shown to exhibitsignal loss in areas of complex flow such as those found in stenotic andregurgitant jets (see for instance ‘Valve and great vessel stenosis:assessment with CMR jet velocity mapping’ Radiology 1991; 178: 229,Kilner P J, Firmin D N, Rees R S O et al). Although not directly relatedto the severity of stenosis or to other pathology, the degree and extentof signal loss has proved to be a qualitative indicator of valvularheart disease and can also aid in the positioning of further imagingplanes for quantitative analysis with phase velocity mapping(‘Diagnostic accuracy and estimation of the severity of valvularregurgitation from the signal void on cine magnetic resonance imaging’Am Heart J 1989; 118: 760-7, Wagner 5, Auffermann W, Buser P et al.).

Due to the requirement for a long echo time however, such a sequencecannot be segmented to allow breath-hold imaging and studies typicallytake approximately 2-minutes to acquire with respiratory motion artefactbeing a frequent problem.

True FISP cine imaging has also been shown to exhibit signal loss inregions of complex flow and has the advantage that the acquisition maybe performed during a period of breath-holding. However, the pattern ofsignal loss with true FISP is different from that obtained with aconventional sequence and appears not to delineate the extended regionof complex flow around the stenotic or regurgitant jet (see for instanceKilner P J, John A, Lorenz C et al. In Proceedings of the 4^(th) annualmeeting of the Society of Cardiovascular Magnetic Resonance 2001,Atlanta, p77. In addition, true PISP is less robust than spoiledgradient echo imaging and has a high sensitivity to fieldinhomogeneities and shimming errors.

The present invention provides a method of performing magnetic resonanceimaging comprising applying a segmented gradient echo sequence withacceleration sensitivity and velocity compensation to a body to beimaged.

In an alternative arrangement, the invention provides a method ofperforming magnetic resonance imaging comprising applying a segmentedgradient echo sequence with velocity sensitivity to a body to be imaged.

The present invention thus provides an imaging technique which resultsin a degree of signal loss similar to that obtained with a conventionalsequence whilst allowing the entire acquisition to be performed in theduration of a single breath-hold.

It is an advantage of the invention, shown particularly in the firstaspect of the invention mentioned above, that signal losses areintroduced into high speed imaging operations which correspond to thelosses previously caused by turbulent flows and seen in lower speedimaging processes.

The invention will be better understood from the following description,given by way of example, of embodiments of the invention, which shouldbe read in conjunction with the attached Figures, in which:

FIG. 1(a) illustrates velocity sensitised (a) sequences for thequalitative assessment of signal loss;

FIG. 1(b) illustrates acceleration sensitised sequences for thequalitative assessment of signal loss;

FIG. 2 illustrates systolic and diastolic frames from cine datasetsacquired using a conventional TE14 sequence ((a) and (d)), thebreath-hold velocity sensitised sequence ((b) and (e)) and thebreath-hold acceleration sensitised sequence ((c) and (f)) in a patientwith mitral regurgitation and stenosis; and

FIG. 3 illustrates systolic frames from oblique coronal and LVOT cinedatasets acquired using a conventional TE14 sequence ((a) and (d)), thebreath-hold velocity sensitised sequence ((b) and (e)) and thebreath-hold acceleration sensitised sequence ((c) and (f)) in a patientwith aortic regurgitation.

Conventional gradient echo sequences with relatively long echo-timesvisualise complex flow as signal loss, the degree and extent of which isa qualitative indicator of valvular heart disease. Since the longecho-time precludes segmentation, breath-hold imaging is not possibleand acquisitions typically take 2-minutes with respiratory motionartefact being a frequent problem. In the present invention, a segmentedsequence is used which generates similar degrees of signal loss to aconventional gradient echo sequence. However, the acquisition durationis reduced and breath-hold imaging is feasible, removing respiratorymotion artefact.

Both velocity and acceleration sensitised sequences were developed andcompared with a conventional sequence in 8 patients with flowdisturbances and in 4 healthy subjects. The image quality of bothbreath-hold sequences was significantly better than that of theconventional sequence (p<0.01). In addition, the image quality achievedwith the acceleration sensitised sequence was significantly better thanthat achieved with the velocity sensitised sequence (p<0.01) whereartefact from beat-to-beat variations in blood flow velocities was afrequent problem

It is concluded that signal loss in complex flow is best demonstratedusing the breath-hold acceleration sensitised sequence where the signalfrom both stationary and constant velocity material is rephased at theecho-time.

This invention was developed using a Siemens Sonata scanner equippedwith gradients having a peak strength of 40 mT/m and a peak slew rate of200 mT/m/ms on each axis independently.

Two sequences were developed, as shown in FIG. 1, both of which werebased on a simple segmented gradient echo sequence. In the first (a),velocity sensitivity was introduced with the addition of a bi-polargradient in both the slice select and frequency encoding (read)directions. Phantom and initial patient studies enabled the velocitysensitivity in both directions to be empirically adjusted to give asimilar degree and extent of signal loss as a conventional gradient echosequence with an echo-time (TE) of 14 ms and velocity compensation inboth slice-select and frequency encoding directions.

In the second sequence (b), the gradient waveforms in the slice-selectand frequency encoding (read) directions were modified to give anacceleration sensitivity, whilst maintaining velocity compensation atthe centre of the echo readout. This involved the addition of extragradient lobes between the slice selection and signal readout (see, forexample, ‘The application of phase shifts in NMR for flow measurement’Magn Reson Med 1990;14: 230-41; Firmin D N, Nayler G L, Kilner P J,Longmore D M), the timing and magnitude of which were such that thephase shifts due to acceleration in both directions were equal to thoseintroduced by the conventional TE14 sequence.

The echo times of the sequences developed in (a) and (b) were 6.9 ms and8.2 ms respectively. Breath-hold segmented k-space acquisitions with 7views per segment and temporal view-sharing (15) enabled the acquisitionof cine data with effective temporal resolutions of 45 ms and 50 msrespectively over 18 cardiac cycles. All studies were performed with a350 mm FOV and 6 mm slice thickness and an acquisition matrix of256×128.

In order to assess the effectiveness of the techniques outlined above,conventional TE14, breath-hold velocity sensitised and breath-holdacceleration sensitised acquisitions were performed in 4 healthyvolunteers (mean age 28 years, range 25-31 years) and in 8 patients(mean age 55 years, range 45-67 years) with flow disturbances, due toeither valvular heart disease (N=5) or to coarctation (N=3). For eachpatient, following standard clinical protocols, the image plane wasselected so as to best demonstrate the flow disturbance. In four of thepatients, acquisitions with all three sequences were carried out in 2image planes, resulting in a total of 12 studies available forcomparison in the patient group. In the healthy volunteer group,acquisitions with all three sequences were performed in three imageplanes—the four-chamber view to demonstrate flow through the mitral andtricuspid valves, and also, the left and right ventricular outflow tract(LVOT and RVOT) views to demonstrate flow through the aortic andpulmonary valves respectively—resulting in a total of 12 studies beingavailable for comparison.

The image quality of the cine data acquisitions were scored by twoindependent observers according to the presence of respiratory and bloodflow artefact, with 0=no or minimal artefact, 1=minor artefact,2=moderate artefact and 3=severe artefact. In cases of disagreement, aconsensus opinion was reached. Friedman two-way analysis of variance wasused to determine whether there were any significant differences in theimage quality obtained with the conventional, the breath-hold velocitysensitised and the breath-hold acceleration sensitised sequences. If so,matched pairs Wilcoxon analysis (with Bonferroni correction for multipletesting) was performed to determine where the differences lay.

For all subjects studied, the conventional TE14, the breath-holdvelocity sensitised and the breath-hold acceleration sensitisedsequences produced similar degrees and extents of signal loss.

FIG. 2 is an example showing the results of using the sequences in asubject with mitral stenosis and regurgitation, the acquisitions beingperformed in the horizontal long axis plane, with the areas of interestin the images identified by the superimposed circles. Systolic frames,showing a regurgitant jet extending into the left atrium, are shown in(a)-(c) and diastolic frames, showing signal loss within the leftventricle, are shown in (d)-(f). FIGS. 2(a) and (d) show systolic anddiastolic frames from the conventional TE14 sequence acquired overapproximately 2 minutes. In this example, the image quality is good andrespiratory motion has not noticeably degraded the image qualityobtained. The corresponding cine frames from the velocity sensitisedsequence of FIG. 1(a) are shown in FIGS. 2(b) and (e) and show a similardegree and extent of signal loss to the conventional sequence. However,although the images are free from respiratory motion artefact, they aredegraded by artefacts from beat-to-beat variations in blood flowvelocity which smear out in the phase encode direction. In contrast,FIGS. 2(c) and (f) which show the corresponding cine frames acquiredwith the acceleration sensitised sequence of FIG. 1(b), where constantvelocity material is rephased at the centre of the echo readout, are ofhigh quality and devoid of both respiratory motion and blood flowartefacts.

FIG. 3 shows the results of using these sequences in a patient withaortic regurgitation, with cine acquisitions having been made in bothoblique coronal ((a)-(c)) and LVOT ((d)-(f)) planes, with the areas ofinterest identified as in FIG. 2. All of the images shown are systolicframes and show complex flow extending into the aorta. However, it isclear that, in this case, the images acquired with the conventional TE14sequence are considerably degraded by respiratory motion artefact. Thoseacquired with the breath-hold velocity sensitive sequence ((b) and (e))are devoid of respiratory motion artefact but artefact from constantvelocity blood is present, although not to as great an extent as that inthe patient shown in FIG. 2. The image quality obtained from using theacceleration sensitised sequence ((c) and (f)) are again superior.

The appearances observed in FIGS. 2 and 3 were similar for all subjectsstudied. Table 1 shows the mean image quality scores for the threesequences used in patient and healthy volunteer groups; both separatelyand combined: Breath-hold Breath-hold Conventional velocity accelerationTE14 sensitised sensitised sequence sequence sequence Patients 2.67 1.830.42 (12 studies) Healthy 2.83 2.0 0.25 Volunteers (12 studies) Combined2.75 1.92 0.33 (12 studies)(Table 1 shows average image quality scores for cine acquisitions usingthe conventional TE14 sequence, the breath-hold velocity sensitisedsequence (FIG. 1(a)) and the breath-hold acceleration sensitisedsequence (FIG. 1(b)). (0=no or minimal artefact, 1=minor artefact,2=moderate artefact and 3=severe artefact). Both breath-hold sequencesgave significantly improved image quality compared to the conventionalsequence (p<0.01) and the acceleration sensitised sequence gavesignificantly better image quality than the velocity sensitised one(p<0.01). This was true for the combined patient and healthy volunteergroup and for the patient and healthy volunteer subgroups.)

The quality of the free-breathing TE14 acquisitions was frequently poor,with severe respiratory motion artefacts being often present. Althoughthe image quality achieved with the breath-hold velocity sensitisedsequence (FIG. 1(a)) was significantly better (mean image score 1.92 vs.2.75, p<0.01), it was generally degraded by blood flow ghostingartefacts. In contrast, the image quality obtained using theacceleration sensitised sequence (FIG. 1(b)) was excellent andsignificantly better than both the conventional and the breath-holdvelocity sensitised sequences (mean image quality score 0.33 vs. 2.75and 1.92 respectively; both p<0.01). It is thought that variations inblood flow velocities from beat to beat can result in phase variationsand associated ghosting artefacts (16,17) when using the velocitysensitised sequence whereas high acceleration and other high orders ofmotion are only present in the region of highly complex flow where theresulting wide range of intra-voxel phase shifts tend to result insignal cancellation.

It will be seen then that this invention provides a segmented sequencewhich generates similar degrees of signal loss in complex flow to aconventional gradient echo sequence. By comparison, the acquisitionduration is considerably reduced (for instance from 128 to 18 cardiaccycles, keeping the same imaging resolution) and breath-hold imaging isfeasible, removing respiratory motion artefact. As discussed, oneapproach to generating signal loss is to add velocity sensitivity to apreviously compensated sequence, as in FIG. 1(a), although in this caseartefacts from flowing blood still degrade the images obtained (FIGS. 2and 3, (b) and (d)). Signal loss is instead best generated by using theacceleration sensitised sequence shown in FIG. 1(b) where bothstationary and constant velocity blood signal are rephased at the centreof the echo readout.

1. A method of performing magnetic resonance imaging on a body, themethod comprising applying a segmented gradient echo sequence to thebody with acceleration sensitivity and velocity compensation.
 2. Themethod according to claim 1 comprising the steps of: applying a firstgradient magnetic field to the region surrounding the body to be imaged;applying radio frequency energy to the body during the application ofsaid first magnetic field whereby to cause nuclei in a designated sliceof the body to resonate; and applying one or more second gradientmagnetic fields to said body at a predetermined time after theapplication of said radio frequency energy, and detecting responsesthereto, said responses being supplied for analysis and imaging of thebody; further comprising providing said acceleration sensitivity andsaid velocity compensation by altering the magnetic field around thebody during the time interval between applying said radio frequencyenergy and applying said one or more second gradient magnetic fields. 3.The method according to claim 2 wherein said acceleration sensitivityand said velocity compensation are provided by applying one or moreadditional gradient magnetic fields corresponding to one ore more ofsaid first and second gradient magnetic fields to said body during saidtime interval.
 4. A method of performing magnetic resonance imaging on abody, the method comprising applying a segmented gradient echo sequenceto the body with velocity sensitivity.
 5. The method according to claim4 comprising the steps of: applying a first gradient magnetic field tothe region surrounding the body to be imaged; applying radio frequencyenergy to the body during the application of said first magnetic fieldwhereby to cause nuclei in a designated slice of the body to resonate;and applying one or more second gradient magnetic fields to said body ata predetermined time after the application of said radio frequencyenergy, and detecting responses thereto, said responses being suppliedfor analysis and imaging of the body; further comprising providing saidvelocity sensitivity by altering the magnetic field around the bodyduring the time interval between applying said radio frequency energyand applying said one or more second gradient magnetic fields.
 6. Themethod according to claim 5 wherein said velocity sensitivity isprovided by applying one or more additional gradient magnetic fieldscorresponding to one ore more of said first and second gradient magneticfields to said body during said time interval.